Light emissive displays provide a very bright and high contrast image when the ambient illumination from the surroundings is very low. When the ambient illumination is high, for example in sunlight, the contrast of the display may be much lower because the ambient illumination is reflected from the display and decreases the contrast of the display.
To some extent, ambient illumination on a display can be accommodated, for example, by making the display brighter. Techniques for sensing the ambient illumination and making a display brighter are known; see for example U.S. Pat. No. 6,320,325 by Cok et al., issued Nov. 20, 2001. However, simply increasing the power to a display is problematic, particularly for OLED displays whose emissive materials age more rapidly as the display gets brighter. However, such a design does not accommodate the need for improved efficiency.
While power efficiency and contrast are always desirable, they are particularly desirable in portable applications because an inefficient display limits the time the device can be used before the power source is recharged. In fact, for certain applications the rate of power consumption may be more important than any other display characteristic with the exception of visibility.
There is a need, therefore, for an improved display having improved power efficiency and contrast.